2-D Electron and Metastable Density Profiles Produced in He - PowerPoint Presentation

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2-D Electron and Metastable Density Profiles Produced in Helium FIW Discharges

B. R. Weatherford and E. V. BarnatSandia National LaboratoriesZ. Xiong and M. J. KushnerUniversity of MichiganSlide2

Fast Ionization Waves (FIWs)

Nanosecond-duration, overvoltage (> breakdown) E-fields  Diffuse volume discharge at elevated pressuresHigh-energy electrons efficiently drive inelastic processes

Ideal for large volume, uniform, high pressure production of:Photons

Charged particlesExcited speciesProposed Applications:

Pulsed UV light sources / laser pumpingHigh-pressure plasma chemistry

Plasma-assisted combustion

Runaway electron generation

2Slide3

Current Understanding of FIWs

Axial FIW propagation studied extensivelyCapacitive probes  Average E-fields, e-

densityOptical emission  2-D profiles, wave speeds

Laser diagnostics  Spatially resolved

E-fields

Radial variations important, but still unclear

Varying

E

-field? Higher density or T

e

? Photons?Applications may require volume uniformityWhat do profiles tell us about the physics?

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Increasing Pressure

Vasilyak (1994)

Takashima (2011)

Positive Polarity

Negative Polarity

Helium FIW, 20 Torr, 11 kVSlide4

Experimental Setup - Chamber

Discharge Tube: 3.3 cm ID x 25.4 cm longHV electrode inside Teflon sleeve, grounded shield

Imaged area: 20-140 mm from ground electrode

Helium feed gasPressure 1-20 Torr

~14 kV (open load) +HV pulses20 ns duration, 3 ns rise time

1 kHz pulse rep rate

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2-D LCIF Diagnostic Scheme

2-D maps of electron densities acquired from helium line intensity ratiosPump 2

3S metastables to 33

P with 389 nm laserElectron collisions transfer from 3

3P

 3

3

D

Image LIF @ 389 nm (3

3

P-23

S) and LCIF @ 588 nm (33D-2

3P) after the laser pulse

Ratio depends linearly on e- density

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Barnat (2009)Slide6

Electron Densities vs. Pressure

Density maps @ fixed rep rate & voltage, 1-16 TorrICCD delay time: 100 ns after FIW, 20 ns windowPeak densities on scale of 1011 cm-3 for all pressuresLow P

 center-peakedHigh P  wall-peaked

Max uniformity, ne at intermediate pressure

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Wavefront Motion

Increasing Pressure

Key Questions:

What causes the transition in e

-

densities?

Can we explain this with a model?Slide7

Metastable Densities vs. Pressure

Helium 23S metastable profiles, 1-16 TorrRelative densities from LIF intensitiesLaser absorption measurements for calibration (B. Yee)Same general trends, but less drastic than

neCenter-peaked to volume-filling / uniform

Similar FIW decay lengths

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Wavefront Motion

Increasing PressureSlide8

Simulation Results - nonPDPSIM

2-D fluid simulationPhoton transportStepwise ionizationPlasma chemistryEEDF calculated from two-term expansion of Boltzmann equation

Same voltage pulse shape as in experiment

Simulations produce similar results as LCIFN

e ~ 1011

-10

12

cm

-3

Trend in radial profiles with variable pressure

Wave velocities ~ cm/ns

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1 Torr Profiles

16 Torr Profiles

(Xiong and Kushner)Slide9

Electrons vs. Metastables

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Experiment: n

e

, N

He*

have different radial profiles @ high pressure

Metastables shifted to center

Model: n

e

, NHe* track each other

Model results rule out:Volume photoionizationPhotoelectrons from wall

n

e

16 Torr Profiles - Simulation

N

He*

Key Questions:Why are these profiles different?

What does this say about FIW physics?He* Profiles - Experiment

(Behind wavefront)

n

e

N

He

*

Top: Experiment

Bottom: SimulationSlide10

E

-field, Effective Te Distributions

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Simulations

 Strong radial

E

near wall

Exceeds runaway e- threshold (~210 Td in He)

Radial

E

exceeds axial E

in and behind FIW front1 Torr: Mean e- energy nearly uniform

E-field fills much of the volume

16 Torr: Mean e-

energy highest at wallE-field drops rapidly away from wall

Electrons cool via collisions

16 Torr – Te and E

1 Torr – Te and E

Axis

Axial & Radial

E

, 16 Torr

Inside wavefront

Wall

Axial & Radial

E

, 16 Torr

Behind wavefront

Axis

WallSlide11

Effect of Runaway Electrons

σiz peaks near 150 eV,

σHe*

at 30 eVRadial fast e- flux in cylindrical geometry

 competing processes:Focusing of e

-

flux, scales as 1/r

Loss of “fast” flux via inelastic collisions

Cooling of fast electrons via elastic collisions

1-D production profiles estimated due to radial runaway e

-

flux from wall

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Electron cooling



separated

e-

and He* profiles

Fixed energy vs.

r

captures pressure trend

Ionization, 2

3

S Cross-sections

30 eV e-, constant energy

4 Torr, with collisional cooling

Initial EnergiesSlide12

Summary

2-D maps of electron and 23S metastable densities in a positive polarity He FIW measured using LCIF/LIFCenter-peaked ne

at low pressure, wall-peaked at high pressureMetastable profiles shift from center-peaked to volume-filling

Intermediate pressures  highest densities and uniformity

2-D fluid simulations capture similar trends in n

e

Peak e- densities of 10

11

-10

12

cm-3; shift in radial profiles

Predicts metastable distributions which track e- densities

Radial E-fields yielding runaway e

- may explain the difference

Runaway electrons are difficult to capture in fluid modelDropoff in

E at high pressure  fast e

- from walls lose energy

High energy  ionization; Lower energy  metastable productionEnergy decay along radius causes spatial separation in profiles

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Thank you!

Questions?Comments?This work was supported by the Department of Energy Office of Fusion Energy Science Contract DE-SC0001939.

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